Optical techniques have been used in the aerodynamics industry for many years to measure temperature and pressure on surfaces of wind tunnel models. Such techniques commonly employ coatings that emit light under ultraviolet or blue light illumination. The intensity of the emitted light is sensitive to pressure and temperature changes in and around the coating. The coatings are mainly of two types, pressure sensitive coatings and temperature sensitive coatings, and usually employ luminophors incorporated into some matrix, such as a polymeric binder. These coatings enable measurement of essentially continuous pressure and temperature field distributions along complex curved surfaces, limited only by the resolution of the imaging equipment used to capture variations in luminous intensity.
The coatings used in the past present certain challenges. First, non-uniform illumination of the surface and variation in coating thickness can give rise to variation in light emission by the coating that is not due to temperature or pressure variation. To remove such effects, a second luminophor that is not sensitive to pressure or temperature is commonly used as a reference. Additionally, sensitive luminophors typically have some sensitivity to both temperature and pressure that have to be decoupled to have an accurate representation of either. A pressure sensitive material is sometimes combined with a temperature sensitive material to provide compensation.
Combining two or more luminophors in a coating introduces other challenges. When multiple luminophors are illuminated by a single radiation field, the emitted light intensity of each luminophor is reduced. Also, one luminophor may emit light that is absorbed by another luminophor, complicating the measurement process. To compensate, the formulation of the coating with the two or more luminophors is typically balanced to provide optimum response, but the optimum may change with temperature and pressure conditions and age of the luminophors. In some cases, the two luminophors may be applied as two separate films, multiplying effects of thickness and concentration non-uniformity in the two films. Moreover, multi-wavelength systems typically have multiple cameras or detectors to register the different wavelengths, and the multiple detectors may have different characteristics that complicate obtaining accurate representations of temperature and pressure.
Coatings containing single luminophors also offer challenges. In systems with no reference luminophor, a first reference image is usually taken at a given temperature considered to be “cold”, a second reference image is taken at a second temperature considered to be “hot”, and the two reference images are normalized to each other. Such systems require frequent re-referencing as the luminophor ages. In some cases, the decay in luminous intensity between light-on and light-off conditions can be used to indicate temperature in a self-referenced procedure, but such procedures are often time-consuming and costly.
There is a need in the aerodynamics industry for improvement in the accuracy, effectiveness, and usability of pressure and temperature sensitive coatings.